Waste heat recovery and conversion system and related methods

ABSTRACT

Various embodiments of a waste heat recovery and conversion system are disclosed. In one exemplary embodiment, the waste heat recovery system may include a heat exchanger for transferring heat from a first fluid to a second fluid and a power conversion unit configured to convert the energy transferred from the first fluid to the second fluid into usable energy. The heat exchanger may include an outer duct defining an inlet and an outlet through which the first fluid flows in and out, respectively, of the outer duct. The heat exchanger may also include an inner duct disposed inside the outer duct and defining an inner channel inside the inner duct and an outer channel between an outer surface of the inner duct and an inner surface of the outer duct. The inner duct may define an internal flow channel through which the second fluid flows to exchange heat energy with the first fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Application Nos. 61/457,995, filed Jul. 29, 2011,61/457,996, filed Jul. 29, 2011, 61/457,997, filed Jul. 29, 2011, and61/457,998, filed Jul. 29, 2011, all of which are incorporated herein byreference in their entirety.

FIELD OF THE INVENTION

Various embodiments of the present invention generally relate to a wasteheat recovery system and related methods. In particular, certainexemplary embodiments relate to a waste heat recovery and/or powerconversion system that can be integrated with a waste heat source.

DESCRIPTION OF RELATED ART

A variety of industrial processes and/or thermodynamic engines dischargewaste heat into the environment. For example, a typical combustionengine used for propulsion of a moving vessel (e.g., a locomotive,automotive, or marine vessel) or power production (e.g., diesel-electricgenerators) has the thermodynamic efficiency of generally less than 40%.Lower efficiencies may result when these engines are operated outside oftheir optimal operational conditions, such as, for example, idling,acceleration transients, and low- and high-power engine operations. Theefficiency can be further decreased for engines with purely mechanicalor unsophisticated fuel metering controls.

For most combustion engine applications, and under most operatingconditions, 22% to 46% of the total energy of fuel used by a combustionengine is normally lost through exhaust gases and engine cooling, whichrepresent waste heat discharged into the environment.

SUMMARY

Thus, there may be a need for developing a heat recovery system andmethod for recovering and/or converting waste heat into useable energy.Recovering such waste heat and/or converting it into usable energy mayincrease efficiency, which results in fuel savings as well as reductionin pollutant emission and thermal discharge into the environment.

Accordingly, various exemplary embodiments of the present disclosure mayprovide an integral waste heat recovery and conversion system andrelated methods capable of reliably and cost-effectively recovering andconverting waste heat energy. For example, certain exemplary embodimentsprovide modular high-pressure heat exchanger for extracting waste heatenergy from various thermodynamic systems and an integral conversionsystem for ultimately transforming the extracted waste heat energy intoelectricity and/or other forms of usable energy.

One exemplary aspect may provide a scalable, modular waste heat energyrecovery and integral conversion system configured to convert waste heatenergy produced by any source that rejects thermal energy into theenvironment, to heat a working fluid circulating within modularhigh-pressure heat exchangers thermally and hydraulically coupled andintegrated with power conversion unit (PCU) for efficient waste heatconversion into usable energy.

The working fluid can be a suitable fluid with thermal-physicalproperties that favor phase changes from sub-cooled liquid tosuperheated vapor when exposed to low-grade heat transfer from any heatsource fluid to the working fluid. The working fluid can also be a gas.In this case, the waste heat recovery and conversion system may besimplified as components dedicated to condensation of the working fluidwould no longer be required.

The modular heat exchangers, all together with the integrated waste heatconversion system, may be configured to match the ever changingthermodynamic parameters characterizing variable waste heat productionsources, especially when these sources are represented by internalcombustion engines.

Another aspect may utilize scalable and modular heat exchangersconfigured to pre-heat and super-heat the working fluid for expansionwithin the integral waste heat conversion system as non-invasiveretrofit for internal combustion engines. In this case, the waste heatrecovery and conversion system may be formed by universal pre-heatinginterfaces coupling the waste heat source thermal-hydraulic system(i.e., pipes, stuck, ducts transporting waste heat fluid) to at leastone turbine expander to a fast alternator and to a high-pressure pumpdedicated to pressurize the working fluid, for the conversion of wasteheat energy into electricity and other usable energy forms. As anexample of usable energy forms, a compressor system may be coupled tothe fast rotating components forming the integral power conversionsystem so as to provide compressed intake air to a combustion engine andincrease its performance while reducing Particulate Matter formation atidling and intermediate power settings.

Although bottom cycle technologies dedicated to combustion enginesgenerally show low efficiencies, high manufacturing cost, highmaintenance costs, and low reliability, the present invention isintended to provide a solution to the low-reliability, and high-costsrepresented by similar technologies by relatively simple to manufacturehigh-pressure heat exchangers with geometries and materials thatwithstand the harsh conditions in which this equipment operates and thatcan be assembled as clusters of heat exchangers, or multiple modules, tomatch the waste heat source availability. The scalable, modular, andintegral thermal-hydraulic connectivity feature of the waste heatrecovery and conversion system characterizing the present inventionallows retrofitting schemes that do not require heavy financing.Individual modules can be installed gradually and in a sequence whereinsavings attained by the operation of each module over time can result in“self-financing” for the installation of additional modules up tomatching the total waste heat source energy availability.

Waste heat energy transported, for example, by the fluid circulating inthe cooling system and exhaust gas tubing of an industrial process or acombustion system heats up a suitable working fluid inside a modularheat exchanger in thermal contact with the fluid transporting waste heatenergy without mixing with these fluid. By the modular heat exchanger,the working fluid expands by changing thermodynamic state from liquid tosuperheated vapor (for working fluid characterized by a system of liquidand vapor, or containing two-phases) within fluid-dynamically optimizedchannels derived internally the high-pressure heat exchanger.

The channels are formed by surfaces within the modular heat exchangerconfigured so as to increase the working fluid residence time and toenhance the working fluid thermal coupling with the fluid transportingwaste heat energy. The residence time is increased by utilizing channelgeometries that force the working fluid through pathways that increaseturbulence while the working fluid accelerates as a result of itsexpansion through the channels and as a result of heat energy transferfrom the high-pressure heat exchanger internal surfaces.

Furthermore, residence time is enhanced by configuring the working fluidand the fluid transporting waste heat energy so as to essentially swirlor rotate the working fluid and the fluid transporting waste heat energywhile wetting and surrounding the surfaces forming the waste heat sourcesystem.

The thermal coupling between the working fluid and the fluidtransporting waste heat energy occurs without mixing and is enhanced byutilizing suitable high thermal conductivity materials that form thesupport structures of the channels so as to make them capable ofwithstanding high-pressure, thermal stresses and mechanical deformationon all axes. As the working fluid travels through the modular heatexchanger, it becomes superheated and, depending on the selected workingfluid, it may change phase from liquid to super heated vapor. At thispoint, the superheated working fluid exiting the modular heat exchangermay enter a series of modular pre-heating and modular heat exchangers soas to increase the waste heat energy transfer to the working fluid, fordirect or indirect expansion of the superheated working fluid vaporswithin at least one set of turbine-alternator systems for the conversionof the working fluid energy into mechanical and electrical energyrespectively.

As mentioned, depending on the application, the modular heat exchangerand waste heat conversion system formed by a turbine and alternator maybe mechanically or thermal-hydraulically coupled to an air compressorsystem for the generation of compressed air. When compressed air isprovided to the intake manifold of a combustion engine, the results arepollutant emission reductions and engine performance enhancement.

Finally, the working fluid exhausting from the turbine system is eithercooled by heat exchangers thermally coupled with environmental fluid(i.e., gaseous single phase working fluid), or made to condense within asudden-condensation chamber (i.e., liquid-vapor phase working fluid),thereby causing a vacuum at the turbine outlet and resulting inincreased waste heat recovery and conversion system efficiency.

Certain exemplary embodiments of the present disclosure focus on bottomcycle applications and make its utilization commercially viable in thecontext of, for example, internal combustion engine applications. Also,various exemplary embodiments may provide the ability of the waste heatrecovery and conversion system to be minimally invasive, with thehigh-pressure heat exchangers sufficiently rugged to withstand fullflame immersion for operation in highly corrosive environments for ahigh-reliability over prolonged periods of time. Overall, the waste heatrecovery and conversion system may efficiently transform low- andhigh-grade waste heat energy into re-usable energy without significantlyinterfering with the fluid-dynamic conditions characterizing the fluidtransporting waste heat energy from the waste heat sources into theenvironment as the high-pressure pre-heating heat exchangers, and thesuperheating high-pressure heat exchangers are designed to reduce backpressure normally generated by drag forming between the heat sourcefluid and the surfaces of the high-pressure heat exchangers.

Additional objects and advantages of the invention will be set forth inpart in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention will be realized and attained bythe elements and combinations particularly pointed out in the appendedclaims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of theinvention and together with the description, serve to explain theprinciples of the invention.

FIG. 1 is a schematic diagram illustrating an application of a heatrecovery and conversion system according to one exemplary embodiment.

FIG. 2 is a schematic diagram illustrating exemplary applications of auniversal thermal and hydraulic coupler forming an interface between thehigh-pressure heat exchanger and the heat source while comprising anozzle to direct waste heat fluid and features allowing thehigh-pressure heat exchanger to mechanically expand/contract freelywhile providing hydraulic sealing with the waste heat source.

FIG. 3 is a perspective view of the universal thermal and hydrauliccoupler forming a high-pressure heat exchanger of FIG. 2.

FIG. 4 is a perspective view of a retrofittable conduit for transportingfluid that carry waste heat energy from a waste heat source into amodular high-pressure heat exchanger integrally coupled to the conduitand showing the exterior walls of a high-pressure heat exchangerfeaturing geometries that allow re-directing of the heat source fluidwhile executing the functions of a nozzle.

FIG. 5 is a perspective view of a retrofittable conduit for transportingfluid that carry waste heat energy from a waste heat source into amodular high-pressure heat exchanger of FIG. 4 showing the coupling ofthe high-pressure heat exchanger with a sealing flexible member.

FIG. 6 is a schematic diagram illustrating exemplary applications of ahigh pressure heat exchanger positioned internally a heat source fluidduct.

FIG. 7 is a perspective view of high-pressure heat exchanger forretrofitting configurations in which the high pressure heat exchangermay be positioned within heat source fluid conduits with minimum dragand maximum heat transfer between the waste heat source and the workingfluid.

FIG. 8 is a perspective view of a waste heat source conduit or manifoldretrofitted with baffles to increase waste heat fluid mixing andturbulence.

FIG. 9 is a perspective view of modular high-pressure heat exchangersgrouped to form a thermal-hydraulically coupled cluster of high-pressureheat exchangers submerged within the heat source fluid.

FIG. 10 is a perspective view of multiple modular high-pressure heatexchangers clustered and thermal-hydraulically connected to universalhigh-pressure heat exchangers with vibrational and structuralde-couplers thermal-hydraulically and mechanically coupled to anexemplary waste heat source represented by the exhaust gases of acombustion engine.

FIG. 11 is a schematic diagram illustrating exemplary applications of apower conversion unit (PCU) for the conversion of recovered waste heatenergy into electricity and other usable energy forms. The schematicillustrates coupling between the expander, a fast generator/motor, ahigh-pressure pump wherein the expander provides features forutilization of compressed air.

FIG. 12 is a perspective view of an exemplary compact power conversionunit with features shown in the schematic of FIG. 11 and offeringuniversal thermal-hydraulic and electrical couplings.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Reference will now be made in detail to the exemplary embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers or letters willbe used throughout the drawings to refer to the same or like parts.

Various exemplary embodiments of the present disclosure provide a systemand method for recovering waste heat from a heat source and convertingit into useable energy. In some exemplary embodiments, the heat recoverysystem may be formed as a single modular system, where variouscomponents of the system are integrated into a single modular unit. Forexample, as will be described in more detail later, the waste heatrecovery and conversion system utilizes a waste heat energy to heat aworking fluid circulating within heat exchangers thermally andhydraulically coupled to an integrated power conversion system formed byone or more turbine expanders housed in a power conversion unit andcoupled to energy conversion systems (e.g., an electric generator, ahigh-pressure pump, a clutch or direct mechanical coupler providingtorque to drive a compressor or as a torque generator).

The working fluid may be any fluid having thermal-physical propertiesthat favor phase changes from liquid to superheated vapor when exposedto a waste heat source. Alternatively, the waste heat recovery andconversion system may utilize a gaseous working fluid. In this case theintegral power conversion unit may be configured to recirculate the gasafter expansion in the expander turbine by substituting thehigh-pressure pump with a compressor/blower and by eliminating thecondenser.

The heat exchangers of the present invention may be utilized topre-heating and superheating the working fluid and as a mechanical andthermal hydraulic interface to decouple the vibrational and structuralenvironment represented by the heat source from the structures of theheat exchangers. The heat exchangers may be formed by compacthigh-pressure heat exchanging surfaces containing channels for thecirculation of the working fluid and provided with universal flanges forthermal-hydraulic coupling with the waste heat source. The heatexchangers may be modular and configured as stand-alone or clusters ofheat exchanger systems all together with the power conversion systemforming the integrated waste heat conversion system of the presentinvention and may be configured to tolerate the stressors generated byever changing thermodynamic parameters characterizing variable wasteheat production sources, especially when these sources are representedby internal combustion engines. To attain the advantages and inaccordance with the purpose of the invention, as embodied and broadlydescribed herein, one aspect of the invention provides means to utilizethe scalable modular heat exchanger and integral waste heat conversionsystems for internal combustion engine applications, wherein the wasteheat recovery and conversion system may be formed by coupling at leastone turbine expander to an electric generator/motor and to an aircompressor for the conversion of waste heat energy into electricity andcompressed air respectively through a configuration that can benon-invasively retrofitted on existing combustion engine platforms, aswell as to new combustion engines utilized for direct propulsion or forhybrid applications (e.g., diesel-electric vehicles, gas-electricvehicles, and stationary combustion-engine driven electric generatorplatforms).

As waste heat sources may be represented by different configurationsutilizing various fluid for the rejection of waste heat energy into theenvironment, an objective of the present invention is to provide auniversal, scalable, modular, waste heat recovery and integralconversion system for the conversion of various forms of waste heatenergy into useful energy easily, with minimally invasivelyconfigurations highly adaptable to various waste heat sources requiringminimum maintenance. Depending on the application, the grade, ortemperature, of the waste heat source (e.g., high-, intermediate-,low-grade) and mass-flow-rate of the fluid transporting waste heatenergy for final rejection into the environment, the scalable modularheat exchanger and integral conversion system of the present inventionmay be coupled in parallel, in series, or any hybrid configuration(e.g., series and parallel). Similarly, the modules forming theembodiment of the invention may be scaled to directly match the wasteheat source availability rating by employing a large single module, orclusters of smaller modules that all together match the total waste heatenergy outputted from the waste heat source.

FIG. 1 is a schematic diagram illustrating various industrialapplications of a heat recovery and conversion system according to oneexemplary embodiment of the present disclosure. As shown in FIG. 1, theconversion of waste heat energy from a heat source 1 into usable energymay result in a lower heat release 44 into the environment as a portionof the waste heat energy normally discharged to the environment isconverted into usable forms of energy. Heat source 1 can be any waste orresidual heat from an industrial process, a combustion engine, or anyother thermal source. By way of examples only, heat source 1 maycomprise exhaust gases from combustion engines, steam or hot gases fromvarious industrial processes, and waste liquids released into theenvironment or cooled down by closed-loop cooling system prior to beingdischarged into the environment.

Heat source fluid 2 may be in the form of gas or liquid. Heat sourcefluid 2, transporting waste heat energy from heat source 1, is made toexchange its thermal energy with 1^(st) Heat Exchanger 3 configured topre-heat working fluid 4 prior to entering into the 2^(nd) Heatexchanger 5 configured to superheat working fluid 4 while transitingwithin its channels. Working fluid 4 circulates in a closed-loop anddoes not mix with heat source fluid 2. 1^(st) heat exchanger 3 and2^(nd) heat exchanger 5 may be configured with a flexiblethermal-hydraulic and mechanical coupling to attenuate vibrationalstressors induced by coupling of the heat exchangers with heat source 1,thereby providing an interface between the heat exchangers and the heatsource to mitigate vibrational and thermal stressors. As heat sourcefluid 2 transfers its thermal energy to working fluid 4, heat sourcefluid 2 lowers its energy content for final discharge into theenvironment at lower temperatures.

The heat exchangers in pre-heating interface 3 may have sufficientlylarge heat transfer surfaces to directly obtain superheating of workingfluid 4. If working fluid 4 is a liquid-vapor phase fluid, working fluid4 may be in a sub-cooled state at the inlet of pre-heating interface 3.Depending on the thermodynamic state of heat source fluid 2, workingfluid 4 may exit pre-heating interface 3 in a sub-cooled liquid, a mixedvapor-liquid, or superheated thermodynamic state.

Working fluid 4 exiting 1^(st) heat exchanger 3 enters the 2^(nd) heatexchanger 5 configured as a stand alone high pressure heat exchanger oras a cluster of modular heat exchangers, to provide additional thermalenergy exchange between heat source fluid 2 and working fluid 4 throughits extended heat transfer surfaces. Superheated working fluid 4 exiting2^(nd) heat exchanger 5 then enters a power conversion unit (PCU) 6 forexpansion within a set of turbines or expander for conversion of heatsource 1 into electricity, compressed air, and/or any other usableenergy forms while providing pumping power for working fluid 4 tocirculate through the closed-loop formed by coupling 1^(st) heatexchanger, 2^(nd) heat exchanger and the PCU 6. PCU 6 may be integral asits expander, pump, alternator/motor, torque coupler and condenser maybe configured as a single piece within the same housing. Thisconfiguration is particularly suitable for applications dedicated tointernal combustion engines coupled to electric generators as the wasteheat recovery and conversion system of the present disclosure converts aportion of the recovered waste heat energy into electricity for readyelectrical voltage and phase coupling with the electrical generators andequipment driven by the internal combustion engine.

The conversion of a portion of the waste heat energy into compressed airmay be required to satisfy pollutant reduction features of the wasteheat recovery and conversion system. Converting a portion of therecovered heat source 1, when applied to combustion engines, intocompressed air provides the combustion engine with excess oxygen (air)when the engine operates at low Revolution per Minute (RpM) and/or athigh transient loads. Most internal combustion engines operating inthese conditions manifest high pollutant emissions. Therefore, providingcompressed air as a result of waste heat recovery and conversion resultsin pollutant emission reductions, while enhancing the combustion engineperformance at low RpM and during transients in which the combustionengine duty cycle is changed from low-to high-loads.

As a result of thermal energy transfer with working fluid 4, heat sourcefluid 2, exiting the 2^(nd) heat exchanger 5, may be characterized bylower temperatures, thereby allowing for Emission Gas Recirculationmethodologies and further decrease pollutant emissions.

For waste heat sources characterized by non air-breathing processes(e.g., requiring compressed air to improve their pollutant emissions),the modular heat exchangers forming 1^(st) and 2^(nd) heat exchangers 3and 5 respectively may be configured to increase working fluid 4 energycontent for expansion within an expander, for example, formed by aturbine-generator system for electricity production only. Forapplications requiring conversion of waste heat energy into mechanicaltorque, working fluid 4 may be expanded through an expander (i.e.turbines) coupled, for example, via gear-box or through a magnetic orhydraulic clutch, to provide shaft work. As working fluid 4 exits theexpander system it enters a condenser 7 integrated with the volumes andsurfaces formed by the power conversion unit housing so as to providecompact thermal-coupling and a vacuum or a low-pressure state at theexit of the expander. This low-pressure thermodynamic state may beinduced by condensation caused by thermal exchange with the compressorfluid (e.g., air). Additionally, auxiliary cooling may be provided byexternal cooling sources as it will be shown in FIG. 11 and FIG. 12.High-pressure working fluid 4 circulates by means of a pump driven bythe torque generated by the expander forming the integral powerconversion unit.

To summarize the exemplary embodiments shown in FIG. 1, the waste heatrecovery and conversion system may comprise a waste heat source 1characterizing by thermal-hydraulic systems (i.e., pipes, ducts, ventingstuck etc.) transporting waste heat energy from heat source 1 to theenvironment, one or more high-pressure heat exchangers (e.g. 1^(st) and2^(nd) heat exchangers 3 and 5) wherein a suitable working fluid 4circulates at high pressure by means of a pump, integrated with anddriven by the power conversion unit 6, for the transfer of waste heatenergy 1 transported by heat source fluid 2 and transferred into workingfluid 4, thereby superheating it, for expansion and conversion intoelectricity and other suitable energy forms, wherein the working fluidcondenses after exiting the expander through a condensing system 7 so asto re-set the closed-loop thermodynamic cycle. 1^(st) and 2^(nd) heatexchangers 3 and 5 respectively all together with the components formingthe PCU 6 may be made integral and modular as these components may behoused as a single piece.

The working fluid may be represented by water which may be used todescribe the exemplary embodiments of the invention. It should beunderstood, however, that any other fluid having suitable thermodynamicproperties may be used alternatively or additionally. For example, forconfigurations wherein working fluid is in a gaseous form, condenser 7may be configured to function as an intercooler while the high-pressurepump integrated with the power conversion unit may be configured tooperate as a re-circulator or blower.

With reference to FIG. 2 and FIG. 3, the operational processes occurringwithin the high-pressure heat exchanger forming a thermal and hydrauliccoupler are described in detail. As shown in this figures, flange 13allows for thermal-hydraulic and mechanical coupling with heat source 1.This provides a thermal-hydraulic and mechanical interface between heatsource 1 and 1^(st) heat exchanger 3 so as to minimize or eliminatethermal and vibrational stressors potentially transferred from the heatsource to the heat exchanger and power conversion unit systems. 1^(st)heat exchanger 3 may be characterized by channels 10 formed by innerjacket walls 18 and outer jacket walls 17. Channels 10 may be configuredto form internal pathways by channel fins 11 for working fluid 4 toincrease its residence time and enhance heat transfer while transitingwithin the 1^(st) heat exchanger. All together, heat channel 10 and fins11 form a structure allowing high-pressure operation.

As heat source fluid 2 transfers energy to channels 10 by thermaltransfer via channel fins 11 and/or via outer and inner jacket walls 17and 18 respectively, without mixing with working fluid 4, thethermodynamic state of working fluid changes from inlet 8 to outlet 9 asit expands and accelerates within channel 10. Depending on thethermodynamic state and mass-flow-rate of heat source fluid 2, and onthe dimensions and materials forming the high-pressure heat exchanger of1^(st) heat exchanger 3, working fluid 4 may exit outlet 9 as sub-cooledliquid single phase, as liquid-vapor two-phase, or as superheated vaporsingle phase. Superheated fluid 21 denotes a single-phase superheatedfluid. If working fluid 4 is gaseous, the gas or mixed gases increasetheir energy content from inlet 8 to outlet 9. As the heat source may beformed by a system inducing vibrational stressors, flexible memberflange 14 allows for mechanical coupling with flexible member 12 whosevibrational decoupling of flange 15 allows for mechanical andthermal-hydraulic coupling with modular 2^(nd) heat exchanger(s) 5without transferring structural loads and vibrational stressesassociated with the system representing heat source 1.

FIGS. 4 and 5 show exemplary geometries of the 1^(st) heat exchanger 3wherein heat source fluid 2 may enter through flange 13 configured tothermal-hydraulically and mechanically couple the 1^(st) heat exchanger3 to heat source 1. As for the embodiments described in FIGS. 2 and 3,the outer jacket walls 17 and inner jacket walls 18 comprise channels 10and fins 11 (shown in FIG. 2) not shown in FIGS. 4 and 5 for simplicity.1^(st) heat exchanger 3 high-pressure inlet 9 and outlet 8 areinterchangeable so as to allow for execution of series, parallel,counter- and parallel-flow configurations according to heat source 1waste energy availability and PCU 6 ratings. FIG. 4 shows a method todirect heat source fluid 2 flow so as to enable retrofitting with avariety of waste heat sources and configurations. In these exemplaryrepresentations, nozzle 16 accelerates waste heat fluid 2 whilere-directing the flow. While flange 14 is mechanically directly coupledto flange 13 (e.g., it may be part of the same body), flexible member 12and flexible member flange 15 allows thermal-hydraulic coupling with2^(nd) heat exchanger 5 while providing a vibration damping system tominimize vibrational and thermal stresses.

With reference to FIG. 6 and FIG. 7, the operational processes occurringwithin the 2^(nd) heat exchanger 5 are described in detail. As shown inthese figures, channels 22 are formed by the jacket-like structurecomprising the superheating inner and outer surfaces 26 and 28respectively, and by internal pathways formed by superheating inner andouter fins 23 and 27.

To minimize drag and reduce backpressure 2^(nd) heat exchanger 5 may beconfigured to feature aerodynamically optimized drag reducing entrance24 and end 25. Additionally, to further reduce aerodynamic drag, 2^(nd)heat exchanger 5 may be configured to be “floating” within a heat sourceduct 20 by providing hydraulic and mechanical connections throughflexible hydraulic couplers 19. The heat source duct 20 may be providedwith the heat source equipment (i.e., exhaust gas manifolds forapplications involving waste heat recovery and conversion fromcombustion engines). Alternatively, a heat source 1 hydraulic conduitmay be formed by configuring hydraulic conduit 20 with flanges 29 formodular coupling with clusters of 2^(nd) heat exchangers 5thermal-hydraulically connected in series, parallel or mixedseries-parallel configurations. As working fluid 4 enters 2^(nd) heatexchanger 5 at inlet 8, its energy content increases due to thermalexchange with heat source fluid 2 and becomes superheated whiletransiting through channels 22. Outlet 9 and inlet 8 areinterchangeable, thus allowing for various counter-flow, parallel-flow,or hybrid parallel-counter-flow configurations.

FIG. 7 illustrates an exemplary embodiment of the 2^(nd) heat exchanger5 without heat source duct 20 for simplicity. In this exemplaryrepresentation, fins 23 may be represented by sealed pins extrudingthrough channel 22 and wetted by heat source fluid 2. In FIG. 7 outerfins 27 are not shown for simplicity. In this representation, superheated fluid 21 exits at outlet 9, while working fluid 4 entering atinterchangeable inlet/outlet 8 is not shown.

With reference to FIG. 8 the exemplary waste heat source duct 20 may beformed by one or multiple conduit, or manifolds with various shapes, fortransport of heat source fluid 2 to the environment. To increase heatsource fluid 2 turbulence and heat transfer the heat source duct 20 maybe retrofitted with mixing baffles 30. For exemplary purposes the heatsource duct 20 may be configured with multiple heat source fluid inletsand outlets that can be coupled to modular heat source ducts via heatsource duct coupling flanges 29.

FIG. 9 is an exemplary representation in perspective view of modular2^(nd) heat exchangers 5 grouped to form a thermal-hydraulically-coupledand mechanically supported cluster of 2^(nd) heat exchangers 5 submergedwithin heat source fluid 2. This figure is not to scale with respect tothe heat source duct 20 represented in FIG. 8. As shown in FIG. 9, heatsource fluid 2 may wet all surfaces of each individual 2^(nd) heatexchangers 5, grouped in the cluster formed by supporting 2^(nd) heatexchangers 5 through cartridge flanges 32. By wetting 2^(nd) heatexchanger 5 outer surfaces 28 and inner surfaces 26 (shown in FIG. 9only for one of the multiple superheating heat exchangers forming thecluster) heat transfer from heat source fluid 2 to working fluid 4 maybe enhanced. Hydraulic connections among each individual high-pressureheat exchanger and those providing one or multiple inlets 8 to workingfluid 4 and outlets 9 to transport superheated fluid 21 may beconfigured with flexible hydraulic couplers 19 shown in FIG. 6, notshown in FIG. 9.

FIG. 10 shows an exemplary embodiment of thermally-hydraulically andmechanically coupled 1^(st) and 2^(nd) heat exchangers 3 and 5respectively, thermal-hydraulically and mechanically interfaced with theheat source 1, supported within modular heat source ducts 20, andsubmerged within heat source fluid 2 resulting from operation of acombustion engine representing, as an example, waste heat source 1. Asshown in this figure while 1^(st) heat exchanger 3 is mechanicallyforming a single piece with the cylinders blocks of the combustionengine, representing as an example waste heat source 1, each heat sourceduct 20 is mechanically linked to waste heat source 1 through flexiblemembers 12, thereby minimizing the impact of vibrations, and that ofexpansion and contractions exerted the materials forming the 2^(nd) heatexchangers 5.

FIG. 11 shows a schematic diagram illustrating exemplary applications ofthe power conversion unit (PCU) 6 for the conversion of recovered wasteheat energy from a heat source 1 into electricity 42 and other usableenergy forms. The power conversion unit 6 may include at least oneexpander 34, mechanically coupled to at least one electricgenerator/motor 36. The electric generator motor 36 may be configured asa fast and compact electrical machine equipped with a coupling shaft.Alternatively all of the rotary components forming the electricalgenerator motor 36, the pump 37, the expander 34, the shaft coupler andthe compressor 40 may mechanically coupled to a single shaft 35. Theexpander 36 may be configured to expand superheated fluid 21 by one ormultiple turbines or positive displacement components. The fast electricgenerator/motor 36 may be configured to produce electrical power whendriven by expander 34 or deliver torque to shaft 35 when operated as anelectric motor. The high-pressure pump may be configured to provide avariable mass-flow-rate (i.e. proportional to shaft 35 revolutions perminute) for example via external control system. The power conversionunit 6 may also be configured to provide mechanical torque resultingfrom recovered waste heat source 1 energy, for example, to drive an aircompressor 40 for combustion engine applications as part of a pollutantreduction system. All of the components comprised in FIGS. 11 and 12 maybe integral and housed to form a single unit.

By hydraulically coupling the power conversion unit 6 to the 1^(st) and2^(nd) heat exchangers the thermodynamic loop shown in FIG. 1 is closed.Working fluid 4 may be configured to be stored within a reservoirintegrally formed within the housing of condenser 7 wherein it issuctioned by pump 37 and compressed for utilization by the 1^(st) and/or2^(nd) heat exchangers 3 and 5 respectively. Superheated fluid 21produced by 1^(st) and 2^(nd) heat exchangers 3 and 5 respectively maybe hydraulically coupled to the power conversion unit 6 by insulatedhigh-pressure tubing (not shown). As superheated fluid 21 expands inexpander 34 it exhausts in the condenser 7. Condensing working fluid 33exiting the expander 34 may undergo condensation by means of active andpassive cooling via thermal exchange with the surfaces forming thehousing of condenser 7 integrated with power conversion unit 6 andtransferring thermal energy to the environment passively via naturalconvection, and/or actively by forced convection through activerecirculation of cooling fluids.

Thermal transfer between the condensing working fluid 33 and thethermodynamic environment represented by condenser 7 may be induced bycirculating the working fluid via condenser auxiliary cooling 49 (e.g.,radiator system), and/or by thermal transfer with a second fluid 41(e.g., air) circulating, for example, via compressor 40 in combinationor independently of the cooling impact induced by enhancing condensercooling fins 48. In this configuration, prior to entering compressor 40,secondary fluid 41 provides cooling to condenser 7 through fins 48.

The electric generator/motor 36 may be configured to mechanically coupleexpander 34 through shaft 35. When the integral power conversion unit 6is configured to recover and convert waste heat source 1 energy fromcombustion engines, the compressor 40 may provide features to reducepollutant emissions while increasing engine efficiency. In thisconfigurations there are combustion engine operating conditions (e.g.,low thermal loads) that may reduce waste heat source 1 ability toprovide sufficient waste heat energy to drive expander 34. To ensurecompressor 40 maintains the function of compressing secondary fluid 41,the electric generator/motor may be actively configured to switch from“generator mode” to “motor mode”, thereby electrically drivingcompressor 40. Compressor 40 represents a usable form of converted wasteheat source. Shaft 35 may be coupled to compressor 40 or any othertorque requiring auxiliary system by shaft coupler 39 which may involvevarious types of clutch systems (e.g., electrical, hydraulic, magnetic,friction and/or centrifugally driven).

Cooling of the electric generator/motor 36 may be accomplished by meanscomprising the generator/motor cooling system 38. These cooling meansmay be particularly required for high compact “fast RpM”generator/motors and may independently or jointly include a thirdcooling fluid 47 to transfer thermal energy with the electricgenerator/motor 36 and its electric interface 43 by electric interfacecooling fins 45, and/or thermal transfer to cooling fluid circulating inthe condenser 7 (i.e., via condenser cooling auxiliary 49), and/orthermal transfer with secondary fluid 41 by electric interface coolingfins 46.

FIG. 12 shows a perspective view of an exemplary power conversion unitintegrating the features shown in the schematic of FIG. 11. As shown inthis Figure, motive and control electric power may be distributed fromand provided to the power conversion unit 6 through electric businlet/outlet 42. Superheated fluid 21 is provided to the integral powerconversion unit 6 through inlet 50, the third cooling fluid 47 may becirculated through inlet/outlet set 51, high-pressure working fluid 4 isprovided at pump 37 outlet 52, condenser auxiliary cooling may becirculated via inlet outlet set 53, and secondary fluid 41 entercompressor 40 suction inlet 54 and exits at compressor 40 discharge 55.The power conversion unit 6 external surfaces may be thermallyinsulated. Thermal-hydraulic and mechanical coupling of the powerconversion unit 6 with 1^(st) and/or 2^(nd) heat exchangers may beprovided through flexible hydraulic couplers 19 and 12 to decouplevibrational and mechanical stresses produced by the heat source 1.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A heat exchanger comprising: an outer duct defining an inlet and anoutlet through which a first fluid flows in and out, respectively, ofthe outer duct; and an inner duct disposed inside the outer duct anddefining an inner channel inside the inner duct and an outer channelbetween an outer surface of the inner duct and an inner surface of theouter duct; wherein the inner duct defines an internal flow channelthrough which a second fluid flows to exchange heat energy with thefirst fluid.
 2. The heat exchanger of claim 1, wherein the first fluidflows in a first direction inside the outer duct, and the second fluidflows in a second direction substantially opposite to the firstdirection.
 3. The heat exchanger of claim 1, wherein both the outer ductand inner duct are substantially cylindrical.
 4. The heat exchanger ofclaim 1, wherein the outer duct and inner duct are substantiallyconcentric.
 5. The heat exchanger of claim 1, wherein the inner ductcomprises a plurality of inner ducts disposed inside the outer duct. 6.The heat exchanger of claim 1, further comprising a plurality of finsextending from the internal flow channel to either the inner channel orouter channel to enhance heat transfer between the first fluid and thesecond fluid.
 7. The heat exchanger of claim 1, wherein the second fluidenters the internal flow channel of the inner duct in a subcooled stateand exits the internal flow channel in a superheated state.
 8. A wasteheat recovery system comprising: a heat exchanger comprising: an outerduct defining an inlet and an outlet through which a first fluid flowsin and out, respectively, of the outer duct; and an inner duct disposedinside the outer duct and defining an inner channel inside the innerduct and an outer channel between an outer surface of the inner duct andan inner surface of the outer duct; wherein the inner duct defines aninternal flow channel through which a second fluid flows to exchangeheat energy with the first fluid; and a power conversion unit configuredto convert the energy transferred from the first fluid to the secondfluid into usable energy.
 9. The system of claim 8, further comprising acondenser configured to condense the first fluid.
 10. The system ofclaim 8, further comprising a preheating interface disposed between aheat source and the heat exchanger, wherein the preheating interface isconfigured to preheat the second fluid prior to entering the heatexchanger.
 11. The system of claim 10, wherein the preheating interfacecomprises a preheating heat exchanger comprising: an inlet through whichthe first fluid enters to the preheating heat exchanger from the heatsource; an outlet through which the first fluid exits the preheatingheat exchanger and enters the heat exchanger; and a preheating channelthrough which the second fluid flows to preheat the second fluid priorto entering the heat exchanger.
 12. The system of claim 10, wherein thepreheating interface comprises a flange configured to fix the preheatingheat exchanger to the heat source.
 13. The system of claim 10, whereinthe preheating interface comprises a flexible member configured toreduce vibration stress between the heat source and the heat exchanger.